More explicit and informative error message for non-rational models

[pogudingleb]

This PR aims at addressing #417
Now, the following errors are shown:

• For D(x1) ~ exp(x1):

ERROR: LoadError: ArgumentError: The system does not seem to have rational (polynomial divided by polynomial) right-hand side.
More precisely: operator exp is not an arithmetic one.

• For D(x1) ~ x1^r1:

ERROR: LoadError: ArgumentError: The system does not seem to have rational (polynomial divided by polynomial) right-hand side.
More precisely: non-integer exponent r1.

• For ode = @ODEmodel(x'(t) = a * exp(x(t)), y(t) = x(t))

ERROR: LoadError: ArgumentError: The system does not seem to have rational (polynomial divided by polynomial) right-hand side.
More precisely: function exp is nonarithmetic one.

• For ode = @ODEmodel(x'(t) = x(t) + b^a, y(t) = x(t))

ERROR: LoadError: ArgumentError: The system does not seem to have rational (polynomial divided by polynomial) right-hand side.
More precisely: noninteger exponent a.